Nowadays, a battery pack, e.g., a Lithium-Ion battery pack, including multiple battery cells is widely used in many electrical products, e.g., hybrid electric vehicle and electric vehicle applications. In general, the battery cells degrade gradually and slowly, and the battery cells degrade differently from each other. As a result, voltages and states of charge (SOC) of the battery cells may be different from each other after multiple charging and discharging cycles, and this difference in degradation leads to unbalances between the battery cells.
More specifically, if the unbalances between the battery cells occur during a charging process, a battery management system may continue to charge the whole battery pack when a battery management system detects a battery cell having the lowest charge is not yet fully charged. As a result, another battery cell having a higher charge may be over-charged. If the unbalances between the battery cells occur during a discharging process, the battery management system may control the whole battery pack to provide power continuously when the battery management system detects a battery cell having the highest charge is not fully discharged. As a result, another battery cell having a lower charge may be over-discharged. Hence, a battery management system may need to move energy from a cell or group of cells to another cell or group of cells to balance the battery cells.
FIG. 1 shows a block diagram of a conventional battery management system 100. As shown in FIG. 1, a battery pack 102 includes multiple battery cells 102_1-102_M. A transformer in the battery management system 100 includes a primary winding 104 and multiple secondary windings 106_1-106_M having the same number of turns. The primary winding 104 is coupled to a switch 108 in series. Each battery cell 102_K is coupled to a corresponding secondary winding 106_K (K=1, 2, . . . , M).
When the switch 108 is turned on, a discharging current IDISCHG flows from the battery pack 102 to the primary winding 104. Energy can be accumulated in a magnetic core of the transformer temporarily. When the switch 108 is turned off, currents I1, I2, I3, . . . and IM are respectively induced in the secondary winding 106_1-106_M and flow to the battery cells 102_1-102_M. Thus, the energy stored in the magnetic core can be released to the battery cells 102_1-102_M. The currents I1, I2, I3, . . . , and IM are reversely proportional to the voltages of the battery cells 102_1-102_M. Therefore (by way of example), if the voltage of the battery cell 102_1 is lower than the voltage of the battery cell 102_2, the battery cell 102_1 can receive more energy than the battery cell 102_2. Since each battery cell 102_K (1≦K≦M) receives energy released from the magnetic core, e.g., even the battery cell having a maximum voltage can receive a corresponding current, the balancing efficiency of the battery management system 100 may be degraded.